A variety of mount assemblies are presently available to isolate vehicle vibrations, such as for automobile and truck engines and transmissions. One of the most popular mounts today is the hydraulic-elastomeric mount of the type disclosed in U.S. Pat. No. 4,588,173 to Gold et al, issued May 13, 1986, entitled "Hydraulic-Elastomeric Mount" and assigned to the assignee of the present invention.
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central passage in the plate. The first or primary chamber is formed between the plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central passage of the plate and reciprocates in response to the vibrations. The decoupler movement alone accommodates small volume changes in the two chambers. When, for example, the decoupler moves in a direction toward the diaphragm, the volume of the portion of the decoupler cavity in the primary chamber increases and the volume of the portion in the secondary chamber correspondingly decreases, and vice-versa. In this way, for certain small vibratory amplitudes and generally higher frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the relatively large central passage, an orifice track with a smaller, restricted flow passage is provided extending around the perimeter of the orifice plate. Each end of the track has an opening; one opening communicating with the primary chamber and the other with the secondary chamber. The orifice track provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler, provides at least three distinct dynamic operating modes. The particular operating mode is primarily determined by the flow of fluid between the two chambers.
More specifically, small amplitude vibrating input, such as from relatively smooth engine idling or the like, produces no damping due to the action of the decoupler, as explained above. In contrast, large amplitude vibrating input, such as heavy engine loading during sudden accelerations or panic stops, produces high velocity fluid flow through the orifice track, and accordingly, a high level of damping force, and desirable control and smoothing action. A third or intermediate operational mode of the mount occurs during medium amplitude inputs experienced in normal driving and resulting in lower velocity fluid flow through the orifice track. In response to the decoupler switching from movement in one direction to another in each of the modes, a limited amount of fluid can bypass the orifice track by moving around the edges of the decoupler, smoothing the transition.
This basic mount design has proved quite successful and represents a significant advance over the prior art engine mounts, particularly of the solid rubber type. Specifically, hydraulic mounts provide a more favorable balance of load support and damping control. It should be appreciated, however, that additional improvement in operating characteristics is still possible. More particularly, it is desirable to provide a mount assembly with a reduced or soft dynamic rate over a selected range to minimize annoying relatively low amplitude/high frequency engine vibrations that would otherwise be transmitted to the passengers in the vehicle.
Present state of the art mount assemblies do not fully compensate for the change in the flow characteristics of the hydraulic fluid that is believed to take place at high frequencies. Specifically, the fluid flowing around the decoupler during switching of movement direction can change from laminar to turbulent flow. This occurs over a range of resonant high frequencies. As a result, fluid flow around the decoupler and through the decoupler passage at these frequencies is restricted reducing the mount's efficiency. Eventually, the flow is effectively choked off, killing the decoupling action. This may result in a significant intermittent pressure buildup in the primary chamber of the mount. Consequently, a very sharp increase in the dynamic rate characteristics of the mount is realized and vibrations are transmitted to the vehicle frame.
The resulting increase in stiffness in particular prevents effective noise suppression and isolation along this resonant range of low amplitude/high frequency vibrations. Thus, the prior art mounts may serve to transmit troublesome, intermittent bursts of noise through the frame of the vehicle to the passengers in the passenger compartment. Thus, a need is identified for a mount assembly exhibiting a reduced dynamic rate by maintaining transition flow around the decoupler in response to virtually all high frequency vibrations, while still maintaining the desired damping for low frequency control of engine displacements.